Saturn's satellites reveal their secrets

• 13 October 2007
• Govert Schilling
• Magazine issue 2625

Iapetus His dark materials

SATURN'S mottled moon Iapetus can blame the sun for its blemished complexion and split personality. Iapetus is a yin-yang satellite, with a black belly and a bright back. The two sides of its character were already evident to 17th-century astronomers, but not the source - which has been a mystery ever since.

Now new clues come from images showing that the dark spots appear mainly on the sunward-facing slopes of craters and mountains. This suggests that they are the sun's handiwork, says the team analysing the images from Cassini. The team released photos of the 1470-kilometre wide moon on Monday at a planetary sciences meeting in Orlando, Florida.

Dark organic-rich gunk, probably from another moon, spatters the side that faces forward as Iapetus orbits Saturn. So the moon seems to pick up gunk much as dead insects accumulate on your car's windshield. Since dark stuff reflects ...